1. Field Of The Invention
This invention relates to series resonant converters for transferring power between a source and a load and more particularly to an improved control system for limiting the voltage within the series resonant converter within a predetermined boundary.
2. Background Of The Invention
A series resonant power converter operates with a high-frequency resonant link interposed between a first and a second switching matrix such as thyristors or the like. Typically, a low frequency power source is connected by the first switching matrix to a series resonant circuit for producing a substantially higher frequency link. The series resonant circuit facilitates the natural current commutation of the first and second switching matrices. In some situations, the series resonant circuit includes a high-frequency transformer for scaling and/or for isolating the source from the load. The series resonant circuit is connected by the second switching matrix to provide a lower frequency output to the load. Filter capacitors are usually connected across the source and the load.
A series resonant power converter can transfer electric power between a source and a load having different voltages, different frequencies and different waveforms including a DC waveform. The direction of the flow of power is reversible between the source and the load.
Series resonant power convertershave been found to be useful for transferring DC to AC as well as providing frequency controlled AC for electrical machines which requires motor speed variations and reversal of the direction of rotation.
Series resonant converters have a number of advantages including natural commutation caused by the zero-crossings of the resonant current. Series resonant converters norma a high link frequency which substantially decreases the size and weight of the input and output filters and any isolating transformer. The high link frequency also substantially decreases noise produced by the series resonant converter. The natural commutation of the series resonant circuit also reduces di/dt related problems and provides excellent reverse recovery performance. The natural commutation of the series resonant circuit allows the frequency of the link to be increased without incurring excessive switching losses.
Unfortunately, series resonant converters operating at high power levels require complex dv/dt protection circuits as well as complex electronic control and protection circuit. In addition, series resonant converters have relatively high snubber losses as well as relatively high switching losses as the result of the use of two current segments per half-cycle applied for stabilizing the stored energy in the resonant network. The disadvantages related to the internal losses of the series resonant converter significantly limits the link frequency which is desired to be as high as possible in order to minimize the input and output filtering requirements.
Therefore, it is an object of the present invention to improve upon the series resonant circuits heretofore known in the art and to provide a series resonant converter having lower losses and increased efficiency.
Another object of this invention is to provide an improved control system for a series resonant converter incorporating the principle commonly referred to as soft-switching.
Another object of this invention is to provide an improved control system for a series resonant converter for limiting the voltage within the series resonant converter.
Another object of this invention is to provide an improved control system for a series resonant converter which substantially reduces the dynamic losses in the first and second switching matrices.
Another object of this invention is to provide an improved control system for a series resonant converter capable of operating at a high link frequency for eliminating the need for low-pass filters.
Another object of this invention is to provide an improved control system for a series resonant converter having a high reaction speed capability and improved stability.
Another object of this invention is to provide an improved control system for a series resonant converter having a controllable four-quadrant operation with synthesize low frequency load waveforms having a very low distortion and high accuracy.
Another object of this invention is to provide an improved control system for a series resonant converter having a substantially constant power factor near unity for a three--phase AC source for substantially all conditions of the load.
Another object of this invention is to provide an improved control system for a series resonant converter having a higher operating efficiency and reliability and with a reduction of stresses on critical components in the series resonant circuit.
Another object of this invention is to provide an improved control system for a series resonant converter having lower harmonic distortion of voltages and currents within the series resonant converter.
The foregoing has outlined some of the more pertinent objects of the present invention. These objects should be construed as being merely illustrative of some of the more prominent features and applications of the invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or modifying the invention with in the scope of the invention. Accordingly other objects in a full understanding of the invention may be had by referring to the summary of the invention, the detailed description describing the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.